With the $1800 at your disposal, it's probably best to splurge it all on good GPU's then.
Then make a dedicated mining machine with those good GPU's like grue suggested.
That will get you the most bang/buck as far as mining is concerned.
Yeah, and from what I'm reading the 5850 is the most efficient miner, and there are (were) 5850's on amazon for $150 each, which means I can buy a LOT of them. Not sure if I have access to this much power haha